A lead-acid unit cell battery and a sealed envelope for containing it and forming a flat accumulator element are already known from European patent application EP 0 296 623 (Sealed lead-acid battery). This document teaches that a lead-acid unit cell comprises positive and negative electrode plates and a sulfuric acid impregnated separator. A flat accumulator element including such a unit cell comprises a flat casing, hermetically sealed with, however, a safety valve for evacuating the gas surplus occasionally produced by electrochemical reactions.
Such a flat accumulator is to solve three main problems. A first problem is that the electrochemical cell is to be enclosed in a substantially flat and thin and sealed envelope, that is to say, an envelope that does not allow external gases to penetrate and does not allow the sulfuric acid to escape which would degrade the performance of the charging/discharging cycles.
According to cited document, this problem is solved by a flat casing containing the elementary electrochemical cell and having a very thin cover sealed by special sealing means.
A second problem is that the flat accumulator is to be very thin. According to cited document, the very thin cover of the flat casing contributes to solving this problem. A third problem is that the electrochemical cell is to be subjected to an internal overpressure so that normal electrochemical reactions occur. According to cited document, this problem is solved by adding to the first casing a second flat stiffening casing which may be made of metal or steel and whose flat stiffening walls show a resistance and sufficient rigidity for withstanding the required pressure while they are thinnest possible.
Worded differently, according to cited document, it is the flat casing and particularly the planar walls of the flat stiffening casing which ensure the function of exerting the necessary pressure for the proper operation of the electrochemical cell. Under these conditions, it is not possible to obtain an accumulator element that has a thickness that is smaller than or about equal to 2 mm, because said planar walls of the stiffening casing would become too thin and could not ensure their function at any rate by using current and reasonably priced materials in the industry.